ISIS is a huge threat to Christianity, and also to civilized culture (including, but not limited to western culture), secularism, principles and ideals. But ISIS isn't the only group that this a threat to said "things". The "moderate" Jihad groups like the FSA, and Nusra are just as bad, especially in the region where they are currently, which is in Syria.
In fact, I think any Muslim who believes Sharia Law should apply to everyone is a threat to Christianity and our way of life.
It has been more than a year since the last of Mosul’s 60,000 Christians were entirely displaced, killed or trafficked at the hands of ISIS. Now, the only Christians remaining in the entire Nineveh Plain of Iraq are those held as sex slaves, and nearly all of the region’s ancient churches and monasteries have been destroyed or converted into a mosque, madrassa or prison. In some places the ISIS genocide has been so thorough that the militants have taken chisels to the crosses engraved on tombstones in Christian cemeteries.
Many Christians have already been murdered at the hands of ISIS. They even go so far as to post the killings on the internet for all to see. ISIS is a threat to the Christians that have already tragically died, and ISIS is a threat to all Christians. They are not quiet about the fact that their goal is to destroy Christianity.
Yes, ISIS is a threat to Christianity and all other religions. They say they practice Islam, but in reality, they are simply terrorists. ISIS is a threat to humanity and every life form on this planet. They will stop at nothing to cause chaos and death and they must be exterminated.
ISIS isn't just a threat to Christianity, but it's a radical entity opposed to any religion or creed it doesn't agree with. ISIS has tortured Japanese citizens, aid workers, other Muslims pretty much anything that it views as Western. It's certainly against Christians as well, but it's hate isn't limited to one religion, race or creed.